each of the handlers Filter1,
Filter2, and
Filter3 will make a call to
$r->filter_input( ), which will return a file
handle. For Filter1, the file handle points to the
requested file. For Filter2, the file handle
contains whatever Filter1 wrote to
STDOUT. For Filter3, it
contains whatever Filter2 wrote to
STDOUT. The output of Filter3
goes directly to the browser.